Mg2+ -dependent ATP occlusion at the first nucleotide-binding domain (NBD1) of CFTR does not require the second (NBD2)

ATP binding to the first and second NBDs (nucleotide-binding domains) of CFTR (cystic fibrosis transmembrane conductance regulator) are bivalent-cation-independent and -dependent steps respectively [Aleksandrov, Aleksandrov, Chang and Riordan (2002) J. Biol. Chem. 277, 15419-15425]. Subsequent to the initial binding, Mg(2+) drives rapid hydrolysis at the second site, while promoting non-exchangeable trapping of the nucleotide at the first site. This occlusion at the first site of functional wild-type CFTR is somewhat similar to that which occurs when the catalytic glutamate residues in both of the hydrolytic sites of P-glycoprotein are mutated, which has been proposed to be the result of dimerization of the two NBDs and represents a transient intermediate formed during ATP hydrolysis [Tombline and Senior (2005) J. Bioenerg. Biomembr. 37, 497-500]. To test the possible relevance of this interpretation to CFTR, we have now characterized the process by which NBD1 occludes [(32)P]N(3)ATP (8-azido-ATP) and [(32)P]N(3)ADP (8-azido-ADP). Only N(3)ATP, but not N(3)ADP, can be bound initially at NBD1 in the absence of Mg(2+). Despite the lack of a requirement for Mg(2+) for ATP binding, retention of the NTP at 37 degrees C was dependent on the cation. However, at reduced temperature (4 degrees C), N(3)ATP remains locked in the binding pocket with virtually no reduction over a 1 h period, even in the absence of Mg(2+). Occlusion occurred identically in a DeltaNBD2 construct, but not in purified recombinant NBD1, indicating that the process is dependent on the influence of regions of CFTR in addition to NBD1, but not NBD2.     

Multiple membrane-cytoplasmic domain contacts in the cystic fibrosis transmembrane conductance regulator (CFTR) mediate regulation of channel gating

The cystic fibrosis transmembrane conductance regulator (CFTR) is a unique ATP-binding cassette (ABC) ion channel mutated in patients with cystic fibrosis. The most common mutation, deletion of phenylalanine 508 (DeltaF508) and many other disease-associated mutations occur in the nucleotide binding domains (NBD) and the cytoplasmic loops (CL) of the membrane-spanning domains (MSD). A recently constructed computational model of the CFTR three-dimensional structure, supported by experimental data (Serohijos, A. W., Hegedus, T., Aleksandrov, A. A., He, L., Cui, L., Dokholyan, N. V., and Riordan, J. R. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 3256-3261) revealed that several of these mutations including DeltaF508 disrupted interfaces between these domains. Here we have used cysteine cross-linking experiments to verify all NBD/CL interfaces predicted by the structural model and observed that their cross-linking has a variety of different effects on channel gating. The interdomain contacts comprise aromatic clusters important for stabilization of the interfaces and also involve the Q-loops and X-loops that are in close proximity to the ATP binding sites. Cross-linking of all domain-swapping contacts between NBDs and MSD cytoplasmic loops in opposite halves of the protein rapidly and reversibly arrest single channel gating while those in the same halves have lesser impact. These results reinforce the idea that mediation of regulatory signals between cytoplasmic- and membrane-integrated domains of the CFTR channel apparently relies on an array of precise but highly dynamic interdomain structural joints.     

Immune effects of mesenchymal stem cells: implications for Charcot-Marie-Tooth disease

Mesenchymal stem cell (MSC) therapy is the most clinically advanced form of cell therapy, second to hematopoietic stem cell transplants. To date, MSC have been used for immune modulation in conditions such as Graft Versus Host Disease (GVHD) and Crohn's Disease, for which Phase III clinical trials are currently in progress. Here, we review the immunological properties of MSC and make a case for their use in treatment of Charcot-Marie-Tooth disease type 1 (CMT1), a group of inherited peripheral neuropathies. CMT1 is characterized by demyelination and aberrant immune activation making this condition an ideal target for exploration of MSC therapy, given the ability of these cells to promote sheath regeneration as well as suppress inflammation. Studies supporting this hypothesis will be presented and placed into the context of other cell-based approaches that are theoretically feasible. Given that MSCs selectively home to areas of inflammation, as well as exert effects in an allogeneic manner, the possibility of an "off the shelf" therapy for CMT1 will be discussed.     

YscU cleavage and the assembly of Yersinia type III secretion machine complexes

YscU, a component of the Yersinia type III secretion machine, promotes auto-cleavage at asparagine 263 (N263). Mutants with an alanine substitution at yscU codon 263 displayed secretion defects for some substrates (LcrV, YopB and YopD); however, transport of effector proteins into host cells (YopE, YopH, YopM) continued to occur. Two yscU mutations were isolated that, unlike N263A , completely abolished type III secretion; YscU_G127D promoted auto-cleavage at N263, whereas YscU_G270N did not. When fused to glutathione S-transferase (Gst), the YscU C-terminal cytoplasmic domain promoted auto-cleavage and Gst-YscU_C also exerted a dominant-negative phenotype by blocking type III secretion. Gst–YscU_C/N263A caused a similar blockade and Gst–YscU_C/G270N reduced secretion. Gst–YscU_C and Gst–YscU_C/N263A bound YscL, the regulator of the ATPase YscN, whereas Gst–YscU_C/G270N did not. When isolated from Yersinia , Gst–YscU_C and Gst–YscU_C/N263A associated with YscK–YscL–YscQ; however, Gst–YscU_C/G270N interacted predominantly with the machine component YscO, but not with YscK–YscL–YscQ. A model is proposed whereby YscU auto-cleavage promotes interaction with YscL and recruitment of ATPase complexes that initiate type III secretion.     

Uptake and metabolism of purine nucleosides and nucleotides in isolated frog skeletal muscle.

When isolated frog skeletal muscles were incubated with ¹⁴C-labeled adenosine, the nucleoside was rapidly taken up by the cells and was either immediately incorporated into adenine nucleotides or deaminated to inosine. Incorporation was predominant at low (micromolar) concentrations whereas, deamination was the major route of metabolism at high (millimolar) concentrations. When muscles were incubated with ¹⁴C-labeled inosine the nucleoside, after entry into the cells, was metabolized to a lesser extent than adenosine. ATP and hypoxanthine were the major products of its metabolism. Intracellular concentrations were calculated using ³H-labeled sorbitol to measure the extracellular space.Because of its lower rate of intracellular metabolism inosine was used to investigate the characteristics of the nucleoside transport system. The uptake of inosine was saturable at high concentrations and was specifically inhibited by the presence of adenosine or uridine in the incubation media. Persantin, a well known specific inhibitor of nucleoside transport, also competitively inhibited inosine uptake, as did theophylline [1, Woo et al. Can J. Physiol. Pharmacol. $\text{52}$, 1063, 1974]. These data, along with the knowledge that in a well-oxygenated muscle, inosine entry follows a downhill chemical potential gradient, strongly support the view that the transport mechanism is facilitated diffusion.The muscle cell membrane does not appear to be permeable to ¹⁴C-labeled ATP under the conditions studied. Investigations of the permeability to the major extracellular degradation products of ATP suggest that AMP was the compound most likely to cross the cell membrane.     

Synthetic chemistry and chemical precedents for understanding the structure and function of acetyl coenzyme A synthase

Acetyl coenzyme A synthase (ACS), found in acetogenic and methanogenic organisms, is responsible for the synthesis and breakdown of acetate. The mechanism by which methylcob(III)alamin, CO and coenzyme A are assembled/disassembled at the active-site A-cluster involves a number of biologically unprecedented intermediates. In the past two years, two protein crystal structures have significantly enhanced the understanding of the structure of the active-site A-cluster, responsible for catalysis. The structure reports spawned a number of important questions regarding the metal ion constitution of the active enzyme, the structure(s) of the spectroscopically identified states and the details of the catalytic mechanism. This Commentary addresses these issues in the framework of existing synthetic and chemical precedent studies aimed at developing rational structure–function correlations and presents structural and reactivity targets for future studies.     

Modulation of juxtamembrane cleavage ("shedding") of angiotensin-converting enzyme by stalk glycosylation: evidence for an alternative shedding protease.

The role of juxtamembrane stalk glycosylation in modulating stalk cleavage and shedding of membrane proteins remains unresolved, despite reports that proteins expressed in glycosylation-deficient cells undergo accelerated proteolysis. We have constructed stalk glycosylation mutants of angiotensin-converting enzyme (ACE), a type I ectoprotein that is vigorously shed when expressed in Chinese hamster ovary cells. Surprisingly, stalk glycosylation did not significantly inhibit release. Introduction of an N-linked glycan directly adjacent to the native stalk cleavage site resulted in a 13-residue, proximal displacement of the cleavage site, from the Arg-626/Ser-627 to the Phe-640/Leu-641 bond. Substitution of the wild-type stalk with a Ser-/Thr-rich sequence known to be heavily O-glycosylated produced a mutant (ACE-JGL) in which this chimeric stalk was partially O-glycosylated; incomplete glycosylation may have been due to membrane proximity. Relative to levels of cell-associated ACE-JGL, rates of basal, unstimulated release of ACE-JGL were enhanced compared with wild-type ACE. ACE-JGL was cleaved at an Ala/Thr bond, 14 residues from the membrane. Notably, phorbol ester stimulation and TAPI (a peptide hydroxamate) inhibition of release-universal characteristics of regulated ectodomain shedding-were significantly blunted for ACE-JGL, as was a formerly undescribed transient stimulation of ACE release by 3, 4-dichloroisocoumarin. These data indicate that (1) stalk glycosylation modulates but does not inhibit ectodomain shedding; and (2) a Ser-/Thr-rich, O-glycosylated stalk directs cleavage, at least in part, by an alternative shedding protease, which may resemble an activity recently described in TNF-alpha convertase null cells [Buxbaum, J. D., et al. (1998) J. Biol. Chem. 273, 27765-27767].